Motorcycle engine

ABSTRACT

In order to increase the displacement of a motorcycle engine, the existing cylinder block of the engine is dismounted from its crankcase, and the existing piston is dismounted from its connecting rod. Then, a new piston having a greater diameter than the dismounted existing piston is mounted on the connecting rod, and a new cylinder block having a greater bore diameter than the existing dismounted cylinder block is connected to the crankcase while sliding the new large-diameter piston along the inner wall of the bore of the new cylinder block. The new cylinder block has a bottom end that is located above the top end of the crankcase when mounted on the crankcase. The new piston is short enough not to be inserted into the crankcase when moved to the bottom dead center. This eliminates the need to disassemble the crankcase or any parts in the crankcase. It is thus possible to easily increase the engine displacement.

BACKGROUND OF THE INVENTION

This invention relates to a method of increasing the displacement of amotorcycle engine by increasing the bore diameter of its cylinder blockand the diameter of its piston, and a piston and a cylinder block usedin the engine of which the displacement has been increased according tothe method of the present invention.

Ordinarily, to increase the displacement of a motorcycle engine, itspiston is replaced with one having a larger diameter, and its cylinderblock is replaced with one having a greater bore diameter or isdismounted from the engine and its bore diameter is increased by boringso that the piston with a larger diameter can be received therein.

As shown in FIGS. 3A and 3B, the cylinder bore 12 extends through thecylinder block 2 from its end facing a crankcase 1 to its end facing acylinder head 7. The cylinder block 2 has its bottom end 2 a inserted inand fixed to the crankcase 1.

A cylinder sleeve may be pressed in the cylinder bore 12 to extend intothe crankcase 1.

The piston 10 has at its lower portion a lateral boss 10 b, and isconnected to the top of a connecting rod 4 by means of a piston pin 15inserted through the boss 10 b and a pin hole 4 a (see FIG. 2, whichitself represents an embodiment of the present invention) formed in thetop end of connecting rod 4 so as to align with the boss 10 b.

The connecting rod 4 is connected at its bottom end to a crank pin 17 ofa crankshaft 3 so that the piston 10 reciprocates between a top deadcenter U shown by phantom line in FIGS. 3A and 3B and a bottom deadcenter D shown by solid line in FIGS. 3A and 3B.

In order to increase the engine displacement, the piston is replacedwith one with a larger diameter, and the cylinder block is replaced withone having a greater cylinder bore diameter or its bore diameter isincreased by boring so as to correspond to the piston with an increaseddiameter. In this arrangement, because the bottom end of the piston 10is adapted to be inserted into the crankcase 1 and the bottom end 2 a ofthe cylinder block 2 is inserted in the crankcase 1, as shown in FIG. 5,corresponding to the piston with an increased diameter the diameter ofthe top opening 1 a of the crankcase 1 has to be increased from A to Bin FIGS. 4A to 4C.

Thus, it is extremely troublesome and time-consuming and costly toincrease the displacement of an engine by increasing the piston diameterand the cylinder bore diameter, because most of the parts of the engine,including the piston 10, cylinder sleeve, connecting rod 4 andcrankshaft 3, have to be disassembled.

SUMMARY OF THE INVENTION

An object of the invention is therefore to provide a method by which thepiston diameter and the cylinder bore diameter can be more easilyincreased.

According to the present invention, in order to increase thedisplacement of the engine, the existing piston is replaced with a newpiston having a greater diameter than the existing piston but having asmaller height than the existing piston, so that the new piston is notinserted into the crankcase. The existing cylinder block is replacedwith a new cylinder piston having a greater bore diameter and sized soas not to be inserted into the crankcase when mounted on the crankcase.

With this arrangement, by replacing or working only members locatedabove the crankcase, the engine displacement can be increased. Thiseliminates the need to e.g. disassemble any parts in the crankcase toincrease the engine displacement.

Also, because there is no need to increase the diameter of the topopening of the crankcase corresponding to the large-diameter piston, thecrankcase maintains high strength.

Specifically, according to the present invention, there is provided amethod for increasing the displacement of a motor cycle enginecomprising a crankcase, a crankshaft rotatably mounted in the crankcase,a first cylinder block connected to the crank case and defining a firstcylinder bore extending therethrough from one end thereof facing acylinder head to another end thereof facing the crankcase, and a firstpiston disposed in the cylinder bore and connected to the crankshaftthrough a connecting rod, the first piston being configured toreciprocate between a top dead center and a bottom dead center; themethod comprising dismounting the first cylinder block from thecrankcase, dismounting the first piston from the connecting rod,mounting a second piston having a greater diameter than the first pistonto the connecting rod, and mounting a second cylinder block defining asecond cylinder bore having a greater diameter than the first cylinderbore on the crankcase, with the second piston received in the secondcylinder bore, the second cylinder block having a bottom end which islocated above a top end of the crankcase with the second cylinder blockmounted on the crankcase, the second piston being configured toreciprocate between the top dead center and bottom dead center andhaving a bottom end located above the top end of the crankcase when thesecond piston is at the bottom dead center.

Thus, it is possible to increase the engine displacement by removing thecylinder head and replacing only members located above the crankcase,i.e. the cylinder block, the piston and their attachments with new ones.There is no need to disassemble the crankcase or any parts in thecrankcase.

The second cylinder block may be prepared by increasing the borediameter of the first cylinder block by boring, or may be a differentmember from the first cylinder block.

The present invention also provides a piston used in the above-describedmethod of the present invention, the piston having a bottom end which islocated higher than the top end of the crankcase when the piston is atthe bottom dead center.

There is also provided a cylinder block used in the method of thepresent invention, the cylinder block having a bottom end which islocated higher than the top end of the crankcase when the cylinder blockis mounted on the crankcase.

According to the present invention, because it is not necessary todisassemble the crankcase or any parts in the crankcase to increase theengine displacement, the engine displacement can be easily increased.Also, because the second piston is not inserted into the crankcase,there is no need to increase the diameter of the top opening of thecrankcase corresponding to the large-diameter piston. The crankcase thusmaintains high strength.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and objects of the present invention will become apparentfrom the following description made with reference to the accompanyingdrawings, in which:

FIG. 1 is a front view of an embodiment of the present invention;

FIG. 2 is a partial exploded perspective view of the embodiment of FIG.1;

FIG. 3A is a front view of an existing engine;

FIG. 3B is a side view thereof;

FIGS. 4A to 4C are a plan view, front view and side view of a crankcase,respectively; and

FIG. 5 is a front view of the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment is now described with reference to the drawings. Thisembodiment is directed to a method of increasing the displacement of aC50E type air-cooled 4-cycle single-cylinder 50 cc engine mounted on a“Supercub” (trade name) made by Honda Motor Co., Ltd. to 95 cc or 106 ccby increasing the bore diameter of its cylinder block. FIGS. 3A and 3Bshow the engine before the cylinder bore diameter has been increased.FIG. 1 shows the engine after the cylinder bore diameter has beenincreased. FIG. 2 is an exploded view of the engine after the cylinderbore diameter has been increased.

The method of the present invention is applicable not only to theabovementioned specific engine but to various other types of engineshaving different displacements.

As shown in FIGS. 3A and 3B, the crankcase 1 defines a crank chamber 16in which the crankshaft 3 is rotatably mounted. The crankcase 1,cylinder block 2 and cylinder head 7 are joined together by means of aplurality of stud bolts 6 through gaskets 13 and 14.

But the crankcase 1, cylinder block 2 and cylinder head 7 may be joinedtogether by any other known means.

The piston 10 reciprocates between the top dead center U and the bottomdead center D by combusting air-fuel mixture introduced into thecombustion chamber, thereby imparting torque to the crankshaft 3 throughthe connecting rod 4.

Now the method for increasing the engine displacement according to thepresent invention is described.

First, the stud bolts 6 are loosened and removed. The cylinder head 7 isthen dismounted from the cylinder block 2, and the cylinder block 2 isdismounted from the crankcase 1. Since the piston 10 is connected to theconnecting rod 4, the cylinder block 2 is dismounted from the crankcase1 while sliding the piston 10 along the inner wall of the cylinder bore12.

Then, a circlip 15 a keeping the piston pin 15 in position is removed,and the piston pin 15 is pulled out to disconnect the piston 10 from theconnecting rod 4.

A new cylinder block 2 having a bore 12 that is larger in diameter thanthe bore of the removed cylinder block 2 is prepared. Also, a new piston10 having a piston ring 11 with a diameter corresponding to the bore 12of the new cylinder block 2 is prepared.

As shown in FIG. 1, the new cylinder block 2 has a bottom end 2 a whichis configured to rest on the edge of the top opening 1 a of thecrankcase 1 without being inserted into the top opening 1 a.

With the top end of the connecting rod 4 protruding from the top opening1 a of the crankcase 1, the new large-diameter piston 10 is connected tothe top end of the connecting rod 4 by means of the piston pin 15 (seeFIG. 2). The new cylinder block 2 is then placed on the crankcase 1 withthe gasket 14 disposed therebetween, while sliding the piston 10 alongthe inner wall of the cylinder bore 12. As shown in FIG. 1, the newlarge-diameter piston 10 has a shorter skirt 10 a than the removedpiston 10 shown in FIGS. 3A and 3B.

Finally, the cylinder head 7 is placed on the cylinder block 2 throughthe gasket 13, and the crankcase 1, cylinder block 2 and cylinder head 7are joined together by the stud bolts 6.

In this arrangement, with the piston 10 at the bottom dead center D, thebottom end 10 c of the skirt 10 a of the piston 10 remains in thecylinder block 2 while not protruding into the crankcase 1. Thus, it isnot necessary to increase the diameter of the top opening 1 a of thecrankcase 1 by boring.

According to the above-described method of the present invention, theengine displacement can be increased simply by replacing the existingcylinder block (first cylinder block) 2 with one (second cylinder block)having a larger bore diameter or increasing the bore diameter of theexisting cylinder block 2, and replacing the existing piston 10 with anew large-diameter piston 10, without the need to dismount the entireengine from the motorcycle.

In the embodiment, in order to increase the engine displacement, theexisting cylinder block 2 is replaced with the new cylinder block 2having an increased bore diameter. But instead, the diameter of the bore12 of the existing cylinder block (first cylinder block) 2 may beincreased so as to correspond to the new large-diameter piston 10 byboring after dismounting it (to form a second cylinder block), if theexisting cylinder block 2 has a sufficiently thick peripheral wall andif the cylinder block 2 maintains sufficient strength after boring.

In this case, when the bore diameter of the existing cylinder block 2 isincreased by boring, its bottom end 2 a, which is inserted in thecrankcase 1, is removed simultaneously. Thus, after boring, the bottomend 2 a of the cylinder case 2 is not inserted in the crankcase 1. Thiseliminates the need to increase the diameter of the top opening of thecrankcase 1 as in the embodiment.

The method of the present invention is applicable not only to theabove-described particular motorcycle engine but to other engines. Thatis, the method of the present invention is equally applicable to both2-cycle and 4-cycle engines, both single-cylinder and multiple-cylinderengines, and engines with different valve structures.

1. A method for increasing a displacement of a motor cycle enginecomprising a crankcase, a crankshaft rotatably mounted in the crankcase,a first cylinder block connected to the crankcase and defining a firstcylinder bore extending therethrough from one end thereof facing acylinder head to another end thereof facing the crankcase, and a firstpiston disposed in the cylinder bore and connected to the crankshaftthrough a connecting rod, the first piston being configured toreciprocate between a top dead center and a bottom dead center, and thefirst cylinder block having a bottom end received in the crankcase suchthat the bottom end of the first cylinder block is located below a topend of the crankcase when the first cylinder block is mounted on thecrankcase, said method comprising: dismounting the first cylinder blockfrom the crankcase, including removing the bottom end of the firstcylinder block from its location in the crankcase; dismounting the firstpiston from the connecting rod; mounting a second piston having agreater diameter than the first piston to the connecting rod; andmounting a second cylinder block defining a second cylinder bore havinga greater diameter than the first cylinder bore on the crankcase, withthe second piston received in the second cylinder bore, said secondcylinder block having a bottom end which is located above a top end ofthe crankcase with the second cylinder block mounted on the crankcase,said second piston being configured to reciprocate between said top deadcenter and bottom dead center and having a bottom end located above thetop end of the crankcase when the second piston is at the bottom deadcenter.
 2. A method according to claim 1, wherein the bottom end of thecylinder block comprises a cylinder block extension part having a partof the cylinder bore defined therein; the crankcase has an openingformed therein; when the first cylinder block is connected to thecrankcase, the cylinder block extension part extends into the crankcasethrough the opening formed therein; and in said dismounting of the firstcylinder block from the crankcase, the cylinder block extension part isremoved from the crankcase through the opening formed therein.
 3. Amethod according to claim 2, wherein in said mounting of the secondcylinder block on the crankcase, the second cylinder block is mounted sothat the second cylinder bore aligns with the opening formed in thecrankcase, and a bottom-most end of the second cylinder block issupported on the crankcase and disposed outwardly of the opening formedin the crankcase.
 4. A method according to claim 1, wherein said secondpiston is shorter in an axial direction than said first piston such thatsaid bottom end of said second piston does not protrude into saidcrankcase when said second piston is at the bottom dead center.
 5. Amethod according to claim 1, wherein a length of said second piston in adirection of reciprocation is shorter than a length of said first pistonin the direction of reciprocation.
 6. A method according to claim 5,wherein the first piston has a first skirt portion on a bottom sidethereof, and wherein said second piston has a second skirt portion atsaid bottom end of said second piston, the second skirt portion of saidsecond piston being shorter than the first skirt portion of the firstpiston such that said bottom end of said second piston does not protrudeinto the crankcase when said second piston is at the bottom dead center.7. A method according to claim 5, wherein the crankcase has a topopening through which the connecting rod extends, the top opening beingsmaller than the second cylinder bore such that said crankcase extendsbeyond said second cylinder block toward the connecting rod.
 8. A methodaccording to claim 5, wherein the crankcase has a top opening throughwhich the connecting rod extends, the top opening being smaller than thediameter of the second piston such that said bottom end of said secondpiston cannot protrude into the crankcase.
 9. A method according toclaim 1, wherein the first piston has a first skirt portion on a bottomside thereof, and wherein said second piston has a second skirt portionat said bottom end of said second piston, the second skirt portion ofsaid second piston being shorter than the first skirt portion of thefirst piston such that said bottom end of said second piston does notprotrude into the crankcase when said second piston is at the bottomdead center.
 10. A method according to claim 1, wherein the crankcasehas a top opening through which the connecting rod extends, the topopening being smaller than the second cylinder bore such that saidcrankcase extends beyond said second cylinder block toward theconnecting rod.
 11. A method according to claim 1, wherein the crankcasehas a top opening through which the connecting rod extends, the topopening being smaller than the diameter of the second piston such thatsaid bottom end of said second piston cannot protrude into thecrankcase.